With a recent demand for a large-capacity network, attention has been paid to a coherent optical communication. This is a communication system in which information is superimposed on a phase or frequency of a light and transmitted, and interferes with a light (hereinafter referred to as “local light”) from a local oscillator located at a receiver side to demodulate a signal.
In the coherent communication system, because a signal light propagated through an optical fiber fluctuates in a polarization direction, there arises such a problem that an interference efficiency of the local light and the signal light is deteriorated, and a receiving sensitivity temporally fluctuates. As means for solving this problem, a polarization diversity detecting system is proposed. This is a method in which the signal light that is in an arbitrary polarization state is divided into two polarization components orthogonal to each other. Each of the polarization components is superimposed on the local light having the polarization directions matched with those of the polarization components to generate the interference light, and a stable receiving sensitivity is realized.
Japanese Patent Application Publication No. Hei3(1991)-251827 discloses a method for implementing polarization diversity detection in which the signal light and the local light are subjected to polarization split, and the respective polarization components are combined with each other by a half beam splitter to generate the interference light.
Japanese Patent Application Publication No. Hei1(1989)-209431 discloses that after the polarization components orthogonal to each other are combined together simultaneously when the signal light and the reference light are subjected to polarization split, the polarization directions match each other to generate the interference light.
Japanese Patent Application Publication No. 2008-92484 discloses that the interference light is generated from a combined light with the help of an optical 90-degree hybrid to conduct phase diversity detection.